1. Field of the Invention
The present invention relates to novel methine dyes and particularly to methine dyes having a pyrazolopyridine nucleus.
2. Description of the Prior Art
Hitherto, water-soluble dyes having a pyrazolin-5-one nucleus have been widely used for photographic sensitive materials because they have excellent absorption characteristics, good compatibility with photographic emulsions and good decoloration properties by sulfites, etc. Namely, they have been widely used as dyes for a filter layer, an antihalation layer or emulsion layers. These kinds of dyes are described in, for example, U.S. Pat. Nos. 2,274,782, 2,527,783, 3,627,532, 3,647,460 and 3,865,817, etc.
However, the wavelengths of light absorbed by dyes having a pyrazolone nucleus are limited even though they have a long methine chain. The dyes which absorb light of a long wavelength are unstable in solution and consequently they are difficult to purify.